extend offset feature to account for architecture/bitness

[williballenthin]

on different architectures (specifically x32 vs x64) the sizes and layouts of structures will differ. right now, capa doesn't have an easy way to limit its offset features based on the architecture, which means there's a greater chance of FP binaries of the wrong bitness. we should consider extending the offset features with tags specifying what bitnesses they match.

initially motivated here: mandiant/capa-rules#54 (comment)

best sketch so far:

- or:
  - offset/x32: 0x24 = pe.szExeFile
  - offset/x64: 0x?? = pe.szExeFile

this would be an easy add, since we could just emit an additional feature for each offset that's tagged with the bitness.

[williballenthin]

@mr-tz points out that we should enable these flavors for numbers, too.

[williballenthin]

after further consideration, suggest using suffixes /x32 and /x64 to better indicate this is an arch thing, not the width of the value, etc.